fix(protocols): a trial parse that raises and is kept as Raw still emits SchemaWarnings #2308

Describe the bug
#1580 ruled that a trial next-layer parse replaced by Raw drops its warnings. That works only when _parse_next_layer does the replacing. When the trial parse raises, hold_warnings.__exit__ replays the held warnings, because it does so for any Exception "whatever discard says". _decode_next_layer then catches the error and keeps the layer as Raw. The warnings describe a layer the result does not contain.

Reproduction

from pcapkit.protocols.link.ethernet import Ethernet; e = Ethernet(bytes(6) + bytes(range(1, 7)) + b'\x08\x00' + bytes.fromhex('4500008c000100004011'))
type(e.payload).__name__   # 'Raw', error='IPv4: header length 20 runs past the end of the data (10 octets)'

It also warns SchemaWarning: packet length < 0: -2, then -6, then -10.

Expected behavior
No SchemaWarnings, as #1580 ruled. The Raw's error already reports the cut header.

System information
Linux, CPython 3.14.9, pcapkit 1.5.0b14 at main 9d504ead5.

Additional context
This conflicts with test_truncated_ipv4_header_warns_whatever_the_on_wire_length in tests/protocols/misc/pcap/test_frame_read_cap_len_2236_unit.py, and with the comment on it in Frame.read. Both pin this leak as the signal that a truncation was noticed (#2236). Options: (1) drop the held warnings when the trial parse raises and is replaced by Raw, and retarget the #2236 test to a signal that survives, such as the Raw's error; (2) keep the leak and narrow #1580's prose to "replaced by _parse_next_layer's length check".

Recommendation: option 1. It is what #1580 ruled, and the truncation stays visible through the Raw.